A touch sensor system has been widely used in various electronic devices such as personal computers (PC), cellular phones, and tablets. In the touch sensor system, a user contacts with a touch panel with his/her finger or a touch stylus to perform an input operation on the touch panel.
For example, in a capacitance type touch sensor system, a change in capacitance (signal value) in the touch panel, which is caused by the contact of the finger of the user or the touch stylus with the touch panel, is detected. Then, the position where the change in the capacitance occurs in the touch panel is recognized as an input position with respect to the touch panel.
In recent years, various techniques have been suggested in order to achieve high accuracy detection of an input position in the touch sensor system. PTL 1 discloses a touch sensor system which corrects the coordinates of the input position that is calculated at the point in time when an input operation is performed on the touch panel by using the coordinates of positions specified by the distribution of capacitance values at a time before the input operation on the touch panel is performed.
Further, it has been known that the change in the capacitance occurs in the touch panel due to an external factor such as bend of the touch panel or exposure of the touch panel to electromagnetic waves (that is, a factor that is different from the input operation on the touch panel). PTL 2 discloses a touch sensor system that determines whether or not the touch panel is bending in order to remove an influence of the change in the capacitance due to bend of the touch panel.
Here, the change in the capacitance due to bend of the touch panel will be described with reference to FIGS. 49(a) and (b). FIG. 49(a) is a diagram that illustrates one example of the distribution of the capacitances in a case where a portion around the center of the touch panel is pressed down and bend occurs in the touch panel. Further, FIG. 49(b) is a diagram that illustrates the distribution of the capacitances illustrated in FIG. 49(a) on larger scale around the center of the touch panel. The values of the capacitance in FIGS. 49(a) and (b) are indicated as relative values that are standardized with a prescribed value (for example, the maximum value of the capacitance) being the reference value.
Here, because the bend of the touch panel is particularly large around the center of the touch panel, as illustrated in FIG. 49(b), a particularly large offset is added to the value of the capacitance. Accordingly, in a case where the amount of offset caused by bend of the touch panel has a similar magnitude to the capacitance caused by the input operation on the touch panel, detection accuracy of the input position degrades like a case where the touch panel mistakenly recognizes that an input is performed in a position where the input operation is not actually performed, and so forth.
In a common touch sensor system, calibrated values of capacitance are used for detection of presence or absence of the input operation and detection of the coordinates of the input position. Calibration means a process of calculating the difference value between (i) the capacitance detected in the touch sensor system and (ii) a reference value, while the value of the capacitance in a static state (for example, the state that the touch panel is embedded in a housing) is set as the reference value. The calibration is conducted, and a static influence (that is, an influence that keeps occurring under the same circumstance) such as bend occurring in a state where the touch panel is embedded in the housing is thereby corrected.
However, in the touch sensor system, the user presses the touch panel down, and the bend amount of the touch panel thereby changes in accordance with the temporal change. Such a state will hereinafter be referred to as dynamic bend of the touch panel. In a case where the touch panel dynamically bends, the detection accuracy of the input position may degrade due to calibration.
The touch sensor system disclosed in PTL 2 removes the influence of the change in the capacitance due to bend of the touch panel by ignoring an input operation in a case where the touch panel dynamically bends.